 So, now that we've gone over the process of mitosis, we are now going to look into the process of meiosis. Remember that earlier we discussed reproduction, and we talked about it on two different levels. We talked about it on the macroscale and the microscale. We went through the microscale in the process of mitosis, how you start off with one cell and create two identical daughter cells, and how this functions in growth and wound repair and asexual reproduction. But what we didn't go into was the macroscale, the obvious, how people make more people, how trees make more trees, the production of offspring. And this is accomplished with a process as known as meiosis. Meiosis is very similar to mitosis, so if you understand mitosis, you already have a leg up on this lecture. However, there are some key differences because instead of starting off with one cell and creating two identical cells, you are starting off with one cell and creating four genetically varied haploid cells. So some important terms to review real quick. I know that we did go over this last time, but this is just a refresher. The first one is chromatin. Remember chromatin is that loosely packed DNA. This is how it normally exists inside of a resting cell, it's so that it can be easily accessible for things like protein formation, etc. When you get ready to divide, chromatin condenses down into a chromosome. You're packing it away in boxes, so that way you don't lose it when you move. Same idea. Sister chromatids, remember that's the chromosome is made out of two sister chromatids, one half of each X, one in black, one in red, joined together by that central point which is your centromere. Homologous chromosomes is a term that we're going to use a lot with meiosis. This is one pair of chromosomes, one from the man, one from the female. You're going to join together and you get a set. This set encodes for genetically similar information. So these are going to be very important to make sure that we align and then divide up in order to create those four genetically varied daughter cells. With mitosis, a human somatic cell, which is a typical body cell, starts off with 46 replicates everything and then divides that up to create two identical daughter cells. Remember that since it has 46, it has two pairs. So it's considered to be diploid or two and in nature. When we talk about sexual reproduction, a male gamete will unite with a female gamete in an event that's called fertilization. During this time, the nucleus of the male gamete melds with the nucleus of the female gamete and you combine the genetic information. If gametes were formed from mitosis, then you would have a male gamete with 46 chromosomes, meaning a female gamete with 46 chromosomes and you'd end up with 92 total chromosomes. That is way too much genetic information for one cell to have. So you'd end up with a 4N cell, 2N plus 2N, 4N cell. This 4N cell is going to have twice as much genetic information as the generation before it and then gametes formed from that generation will create offspring that has twice as much as the generation that it came from previously. So you can see the amount of genetic information is really going to begin to get out of control and you're going to end up with a lot of mutations and probably even a species that's not even viable. So using that logic, we can kind of rule out that there's no way that gametes are formed by mitosis. There has to be a different process that creates these gametes. So the question that we are left with, so how are they formed? They're formed with the process known as mitosis. This is a reductive division and that's a very important term to remember, reductive division. This is where you start off with one cell and you reduce the amount of genetic information so that you create four varied haploid cells. Your haploid is going to have half the number of chromosomes. Diploid had two homologous chromosomes or one set. Haploid only has one chromosome or one half of a set. That way when the male and female haploids unite during fertilization, you are going to restore that two end state and then from there that two end diploid cell can perform mitosis over and over again until you create human beings. Now since myosis only occurs to form gametes, it's only happens in reproductive structures. So you only find it happening in the ovaries and the testes nowhere else in the body. So just like mitosis, meiosis is going to start in the process of interphase. During interphase, you have G1, which is where the cell grows. You have the S phase, remember S for synthesis of DNA, where the two end cell is going to replicate all of its genetic information and become a four end cell. Which means it has four copies. After you synthesize the DNA, you finish up G2, which is the growth and development phase preparing for division. When you look at meiosis, meiosis is a much larger piece of the pie than mitosis. Because unlike mitosis, where the process begins over and over again with each daughter cell formed, meiosis begins with interphase and it ends. Those haploid cells do not go on to divide and reform. They are now gametes that will be released for fertilization events. So there is a beginning and an ending point for meiosis unlike mitosis. So the phases of mitosis and meiosis are the same. They're in the same order that we've already previously learned. Please make another try or I pass my anatomy test. Remember prophase, metaphase, anaphase, telephase. The difference here is that mitosis started off with one cell and created two identical daughter cells from one cell division. With meiosis, you start off with one cell and go through two consecutive cell divisions to create four genetically varied haploid cells. So the process of meiosis is broken up into two individual stages. You have meiosis one and meiosis two. During each of these stages, you go through prophase, metaphase, anaphase, and telephase. But the stage will be dictated by the number that follows. So you go through meiosis one first and then meiosis two. So the first part of meiosis one is prophase one. During this time, the events that happen are very similar to mitosis, where the chromatin is going to condense down to form chromosomes. Remember chromatin is going to be that loosely configured DNA. And you're going to condense it down and pack it into boxes so that you don't misplace any during division. It is important when we talk about this to kind of review something that we've talked about earlier. Remember that during the S subphase of interface, your two end cell replicated itself to become a four end cell. So it has four copies or four sets of your overall DNA. So all of that chromatin condenses down to chromosomes. So you have a lot of genetic information in these cells. That's the same thing that you saw with mitosis, where the two end became four end. But it all has to be condensed down and packaged so that you don't misplace any. Once it is packaged away, the nuclear envelope disintegrates. And the centrioles create spindle fibers, which extend out toward your homologous chromosomes and to each pole so that you can create the elongation effect. These spindle fibers are going to reach the center of your homologous chromosome pairs. Each pair, one in red and one in blue, is known as a tetrad. A tetrad is a pair or a set of homologous chromosomes. Each one will go to a opposite pole during meiosis one. There is something very, very important that happens during prophase one that doesn't happen during mitosis. And this is what's known as crossover. During prophase one, your homologous chromosomes actually come in physical contact with each other. And because they come in contact with each other, they can swap genetic information, which is very neat. It doesn't happen in mitosis. So this chromosome was from the mom. This was from the dad. Both contain genetically similar information, but just slightly variable because everybody's different. That area that touched in the chromosomes and has been exchanged now means that the mother's chromosome contains part of the father's DNA and vice versa. So this physical contact and crossover event allows that swapping of genetic information, which, when we add this to what we know about Mendel and his laws for a chromosome assignment, how it's random, there's trillions of possibilities for gamete production. Trillions of possibilities that lead to genetic variation, which further leads into evolution of a species. So now that we've talked pretty in-depth about prophase one, let's go into metaphase one. During metaphase one, the tetrad that we talked about earlier aligns at the equator, where you have one homologous chromosome, which is facing either pole. The spindle fibers are going to attach to the centromere of your homologous chromosome, also attaching at the pole of the cell. And then you enter into anaphase one. Remember, anaphase is where the spindle fibers work that double duty, where they shorten to pull the chromosomes apart, but elongate at the poles to help elongate the cell. Remember, the further apart you get the two distinct poles, so you can have two distinct cells. And then you have telophase one. During telophase one, the homologous chromosomes are going to arrive at either pole, and this is going to be the reverse of prophase in some species. In some species, it goes through the trouble of reforming the nuclear envelope, dividing up into two separate cells, unwinding all of your chromosomes back into chromatin, and then having a pause before entering the second phase of meiosis. In most species, this doesn't happen. In most species, it's going to be an immediate following, where once the chromosomes reach their destination, instead of having reformation, you move directly into cytokinesis. After that happens, you move into the second phase of meiosis, which is known as meiosis two. During meiosis two, you have prophase two, which is where any chromatin that has unwound begins to condense back down. If a nuclear envelope was formed, it disintegrates again. Spindle fibers, once again, extend out from the centromeres toward those chromosomes. With metaphase, the chromosomes are lined up at the equator. You have the spindle fibers attaching to their centers of the sister chromatids, leading into anaphase, once again, double duty, extending of the cell, causing elongation, and separating those chromosomes so that sister chromatids are being pulled to either pole. And then you end up with telophase two, which is the last part of your meiosis two. During this time, the sister chromatids arrive to their poles, the nuclear envelope reforms, the spindle fibers retract, cleavage furrow forms, and two cells are pinched off. So you end up with four, what will be genetically varied, daughter cells, all of which contain half the number of chromosomes or only one set, and therefore considered to be haploid in nature. This shows you kind of the overview in just one possible combination. Remember that the two end cell replicates everything to become four end. Those are seen as homologous chromosomes. During prophase, you have crossing over events that occur, which you can see by that part of the red has been exchanged with the blue. During meiosis one, you have two cells formed with their chromosomes. During meiosis two, everything's been halved and you just have sister chromatids. So in summary, meiosis is the process by which organisms that replicate with sexual reproduction produce gametes. These gametes unite during fertilization in order to restore the diploid state and have enough genetic material to be viable. Meiosis also provides an opportunity for crossing over, which lends to genetic variability and micro evolution. Micro evolution is important for the viability of a generation and the viability of species. My challenge to you is to see if you can go through the process of meiosis one and two and correctly diagram these processes using the correct terminology. If you're able to do this, then you've conquered meiosis. Congratulations, good luck, thanks.